Replaced speedometer and now overdrive not working.

[mweiman]

I have a 91.5 w250 auto trans that I sold to a neighbor. Anyhow the speedometer was not working but the overdrive shifted fine. I checked out all the wiring and replaced the VSS. Speedometer/odometer still did not work. So I modified the dash cluster and put in an autometer 1480 speedometer. I wired it in and powered and ground from the three wires that the old speedometer connected to. After doing this the overdrive does not work, shifts fine otherwise. Got the speedometer calibrated and it works fine. My question is does the speedometer feed a signal to the pcm or something to allow overdrive to engage. The 3 wires to the old speedometer seemed to be power, ground and signal from the vehicle speed sensor, and I wired the new one in the same. The new speedometer does have an output pin to go to an ecm if needed. I did check over all the wiring before replacing the speedometer, I have rechecked and can't find where anything was left unplugged. I do have power to the overdrive solenoid also.

 

[mweiman]

I ended up wiring some jumper wires in and splicing the old speedometer in with the new one and tucking it in behind the dash cluster and now overdrive works fine. Still don’t understand because it seems to be power, ground, and signal wire to the speedometer. Anyway everything works as it should now.

 

[mweiman]

Here is how the new speedometer turned out. Cut a hole in the dash cluster cover and then painted it black in the middle.